Error Handling Changes (4 to 5.2)

slapi-plugin.h now includes ldap_msg.h, which lists unique
error numbers used by Directory Server.

Functions for dealing with back ends that return int error
codes can now also return SLAPI_FAIL_RETRY. The old values
are also still used.

Directory Server now distinguishes between two results. SLAPI_BIND_FAIL indicates that the bind failed and the server prepared a result
to send. SLAPI_BIND_ANONYMOUS indicates that the bind succeeded
as an anonymous bind.